There are various radar devices that are mounted in, for example, the front side of a vehicle and that detect a target by transmitting a transmission wave to a predetermined detection area containing the front of the vehicle, receiving a reflected wave from the target within the detection area, and thus detecting the target. Many such radar devices employ the frequency modulated continuous-wave (FMCW) system in the vehicle field.
As illustrated in, for example, Patent Document 1 and Patent Document 2, an FMCW radar device uses a transmission signal having a triangular wave shape having alternate rising modulation sections at which the frequency of the transmission signal gradually increases and falling modulated sections at which the frequency of the transmission signal gradually decreases. The radar device calculates the beat frequency in the rising modulated section and the beat frequency in the falling modulated section. Here, the beat frequency indicates the frequency in which the frequency of a transmission signal and the frequency of a reception signal responsive to that transmission signal are mixed. The radar device calculates the relative velocity of a target from the difference between the beat frequency in the rising modulated section and the beat frequency in the falling modulated section.    [Patent Document 1] Japanese Patent No. 3622565    [Patent Document 2] Japanese Patent No. 3575694
However, the methods described in Patent Documents 1 and 2 always have to calculate the beat frequency and the Doppler frequency to calculate the relative velocity, so processing is inevitably complex.
Also, if a plurality of targets are present in a detection area, a plurality of spectrum peaks of the beat frequency exist and thus it is necessary to pair appropriate spectrums. Unfortunately, this pairing process may have an error. If an error occurs, it is difficult to accurately calculate the relative velocity.